1. Field of the Invention
The present invention relates to an image sensor, and more particularly to an image sensor of which productivity and sensitivity are improved.
2. Description of the Related Art
Conventional image sensors are manufactured in a type of charge coupled device (CCD) or complementary metal-oxide-semiconductor (CMOS) image sensor, which are shown in FIGS. 1 and 2.
FIG. 1 is a cross-sectional view showing a structure of a unit pixel in a conventional CCD image sensor.
Referring to FIG. 1, the unit pixel of the conventional CCD image sensor includes: an oxide film 4 formed upon a semiconductor substrate 7; a photodiode N-type region 2 formed within the semiconductor substrate 7 at a predetermined depth; a photodiode surface P-type region 5 disposed on the photodiode N-type region 2 and having an interface with the oxide film 4; a floating diffusion region 3 disposed within the semiconductor substrate 7 and having an interface with the oxide film 4, which is spaced apart from the photodiode surface P-type region 5 by a predetermined distance; a gate electrode 1 disposed on the semiconductor substrate above the floating diffusion region 3; and a P-type well region 6 containing the photodiode N-type region 2, the photodiode surface P-type region 5 and the floating diffusion region 3.
In the CCD image sensor, a photoelectric conversion of an incident light into an electric signal is performed in the photodiode N-type region 2 to generate a signal charge. The signal charge is introduced into the floating diffusion region 3 for amplification after horizontally traversing the P-type well region 6 through the photodiode surface P-type region 5. Thereafter, the amplified signal charge is converted into a voltage signal, and the CCD image sensor outputs the voltage signal.
Generally, the gate electrode 1 is overlapped with the floating diffusion region 3, and is made of doped polycrystalline silicon.
In case the photoelectric conversion of the incident light into the electric signal is performed in the photodiode N-type region 2 to generate the signal charge, an electron is generally used as the signal charge in consideration of a transfer rate.
Further, to maximize a transfer efficiency of the signal charge, the floating diffusion region 3 is usually doped with an N-type impurity.
The oxide film 4 formed upon the semiconductor substrate 7 is a layer for protecting and simultaneously insulating the semiconductor substrate 7, which is generally made of silicon dioxide (SiO2).
The photodiode surface P-type region 5 is disposed on an upper portion of the photodiode N-type region 2 to prevent an appearance of the electrons easily generated and dissipated at a low potential due to an electronic defect on a surface of the semiconductor substrate 7.
Since a voltage value in the photodiode of the CCD image sensor is about 3 volts, the signal charge remains within the photodiode of the CCD image sensor. Therefore, to discharge these excess charges, an N-type semiconductor substrate is disposed below a lower portion of the P-type well region 6.
As described above, in case of the CCD image sensor, the photodiode surface P-type region 5 as a p-channel should be injected into an upper portion of the photodiode so as to remove an internal noise, and the P-type well region 6 as well as an n-channel based semiconductor substrate should be included so as to discharge the excess charges. As a result, the CCD image sensor has a problem that the manufacturing process and structure of the CCD image sensor become complicated.
FIG. 2 is a cross-sectional view showing a structure of a unit pixel in a conventional CMOS image sensor.
Referring to FIG. 2, the unit pixel of the CMOS image sensor includes: an oxide film 4 formed upon a semiconductor substrate 8; a photodiode N-type region 2 formed within the semiconductor substrate 8 at a predetermined depth; a photodiode surface P-type region 5 disposed on the photodiode N-type region 2 and having an interface with the oxide film 4; a floating diffusion region 3 disposed within the semiconductor substrate 8 and having an interface with the oxide film 4, which is spaced apart from the photodiode surface P-type region 5 by a predetermined distance; and a gate electrode 1 disposed upon the semiconductor substrate between the floating diffusion region 3 and the photodiode regions 2 and 5.
In the CMOS image sensor, a photoelectric conversion of an incident light into an electric signal is performed in the photodiode N-type region 2 to generate a signal charge. The signal charge is introduced into the floating diffusion region 3 through the photodiode surface P-type region 5 for amplification. Then, the amplified signal charge is converted into a voltage signal, and the CMOS image sensor outputs the voltage signal.
Generally, the gate electrode 1 is aligned with the floating diffusion region 3 and the photodiode regions 2 and 5, which is made of doped polycrystalline silicon.
The alignment of the gate electrode 1 with the floating diffusion region 3 and the photodiode regions 2 and 5 means that a region of the gate electrode 1 is not overlapped with that of the photodiode regions 2 and 5.
In case the photoelectric conversion of the incident light into the electric signal is performed in the photodiode N-type region 2 to generate the signal charge, an electron is generally used as the signal charge in consideration of a transfer rate.
Further, to maximize a transfer efficiency of the signal charge, the floating diffusion region 3 is usually doped with an N-type impurity.
The oxide film 4 formed upon the semiconductor substrate 7 is a layer for protecting and simultaneously insulating the semiconductor substrate 7, which is generally made of silicon dioxide (SiO2).
The photodiode surface P-type region 5 is disposed on an upper portion of the photodiode N-type region 2 to remove a noise caused by a depletion region of the photodiode and a heat noise caused by a recombination of electrons generated on a surface of the photodiode region or a movement of electrons to the photodiode N-type region 2.
Since a voltage value in the photodiode of the CMOS image sensor is about 1 volt, there exists no signal charge within the photodiode of the CMOS image sensor. Therefore, unlike the CCD image sensor, a well region for discharging excess charges is not needed, so that a P-type semiconductor substrate is provided and an additional well region need not be formed thereon.
As described above, in case of the CMOS image sensor, the photodiode P-type region 5 as p-channel should be injected into an upper portion of the photodiode so as to remove an internal noise. Thus, CMOS image sensor also has a problem that the manufacturing process and structure of the CMOS image sensor becomes complicated.
Further, although research and development for minimizing a leakage noise or a heat noise generated in the CCD or CMOS image sensor have been steadily carried out, the leakage noise or the heat noise is not reduced to a desired level.
The present invention solves the problems of the prior art, and provides an image sensor with improved productivity and sensitivity.
This object, other incidental ends and advantages of the invention will hereinafter appear in the progress of the disclosure and as pointed out in the appended claims.
According to one aspect of the invention, the present invention provides an image sensor comprising a plurality of unit pixels. Each unit pixel includes an oxide film formed upon a semiconductor substrate, a gate electrode formed on the oxide film, a photodiode N-type region formed within the semiconductor substrate and interfacing with the oxide film, the photodiode N-type region being spaced apart from the gate electrode by a predetermined distance and being disposed on one side of the gate electrode, and a floating diffusion region formed within the semiconductor substrate and interfacing with the oxide film, the N+-type region being spaced apart from the gate electrode by a predetermined distance and being disposed on the other side of the gate electrode.
It is preferable that each unit pixel is isolated by an isolation region, which includes a P-type impurity region formed within the semiconductor substrate and having an interface with the oxide film, and a light shielding layer disposed on an upper portion of the P-type impurity region, which is formed of opaque insulating material. More preferably, the shielding layer is made of silicon dioxide (SiO2).
Since the image sensor of the present invention requires less masks than a conventional image sensor in the wafer foundry, the production cost will be reduced. Further, as a process of injecting the P-type impurity is not needed, the manufacturing process is simplified and the productivity is improved.
Furthermore, since a leakage noise or a heat noise can be remarkably reduced while keeping a constant aperture ratio, the sensitivity is improved.